Perforation bonding of booster propellant



May 23, 1961 w. T. MCMICHAEL PERFORATION BONDING 0F BOOSTER Filed Sept. 1, 1955 PROPELLANT INVENTOR.

W. T. M MICHAEL EL M ATTORNEYS PERFORATION BONDING OF BOOSTER PROPELLANT Wallace T. MeMichael, McGregor, Tex., assignor to Phillips Petroleum Company, a corporation of Delaware Filed Sept. 1, 1955, Ser. No. 531,875

Claims. (Cl. 86-4) This invention relates to means for mounting a solid, rocket booster charge by bonding the charge to support rods.

In the course of rocket development, large rockets have been found to consume a major portion of their fuel in getting off the ground. One expedient devised to overcome the large space requirements for this takeofi fuel has been the provision of a launching device equipped with a booster charge designed to carry and accelerate the rocket along a launching ramp. Such launching devices have been successful in launching relatively small rockets such as guided missiles but the pay-load-mile per pound of fuel is extremely low. One deterrent to scaling up the size of the rockets has been in the length of the ramp required for boosting these missiles. It has now been found practical to use short take-off ramps by utilizing a booster charge designed to accompany the missile for a considerable distance in its initial flight. These boosters have a very short (2-10 seconds) burning duration and comprise a large fuel section which is released from the missile proper at the end of the burning period and can be returned for reuse.

Booster charges adapted to be attached to a rocket, as above described are usually attached to a support rod so as to be properly positioned in the rocket case. It is important that the charge maintain its position in the case during the firing of the rocket. In the fabrication of large booster charges one major problem has been the provision of a bond between the booster charge propellent grain and the support rod which will prevent the grain from being torn from the rod as a result of the great acceleration developed during the firing of the booster charge.

It is therefore an object of this invention to provide a means for bonding a solid propellent grain to an internal support rod so as to Withstand the forces of acceleration.

It is another object to provide a method for bonding a perforated, external burning propellant charge to a support means positioned in the perforation.

It is still another object of this invention to provide means for the simultaneous bonding of the support rod and the restrictor to an external burning solid propellant charge.

It is still another object to provide a means for utilizing a rubber restricting-bonding agent in bonding a solid propellant charge to the restrictor and the support rod.

Other objects and advantages will be apparent to one skilled in the art upon study of this disclosure and the appended drawing wherein:

Figure 1 illustrates a perspective of an extruded, slabshaped grain with two support rods and end restrictors in position,

Figure 2 shows an end View of an extruded propellant slab with two perforations,

Figure 3 shows a diagrammatic view of a support rod of this invention,

Figure 4 shows a partial cross-section through the Patented May 23, 1961 grain of Figure 1 taken along the axis of one of the support rods along line 4-4, and

Figure 5 shows a view of an end plate employed in positioning the support rods and the end restrictors.

Several embodiments of the invention can be practiced without departing from the broad scope of this invention. Reference will now be made to the drawing wherein Figure 3 shows one embodiment of the support rod 1 having an axial cavity 2 which is penetrated by radial drill holes from the outer surface of the rod 1. Although the drill holes are shown as arranged with a multiplicity of drill holes in each of several transverse planes at a plurality of points along the axial cavity, these drill holes can also be positioned in a helical spacial arrangement along the outer surface of the rod *1. The invention according to the embodiments described has the advantage that the support rod is in its final position when the restrictor formulation is placed into position.

Figure 4 shows a partial cross-section of the propellent grain and support rod with the support rod in position so that an annular space 4 is provided between the support rod 1 and the propellent charge 5. The support rod 1 is placed in its final position and the bonding material is forced through the support rod and into the annular space a so as to form a network composed of bonding agent, binding the rod and the propellent charge together. in order to completely fill the perforations in the rod and the annular spacewith the bonding agent and to prevent the occurrence of air-filled voids, a tightfitting sleeve 7 of thin metal or other rigid material is positoned over the rod 1. This sleeve has a flared end section 8 which just engages the surface of the propellant perforation when inserted into the perforation around the rod 1. If desired the wall thickness of the sleeve can be substantially equal to the thickness of the annulus 4. The voids in the support rod are filled from the top of the rod as shown in Figure 4 by means of an adapter 9 using an extruding device (not shown) such as a plunger-cylinder pressure gun of the Alemite type or similar hydraulic, pneumatic or mechanical displacement means. The sleeve 7 is placed in position with the flared end adjacent the bottom of the rocket charge perforation as shown in Figure 4 before the extrusion of the bonding agent is commenced. Air is displaced from the axial cavity 2 as the bonding agent is forced in, by means of vent 10, as shown in Figure 3. When bonding agent appears in the vent 10, a counter sunk plug 11 is inserted. Although the vent '10 is shown as a radial drill hole, it can be the end of the axial perforation through support rod 1 or located at some other convenient location.

The filled support rod 1, with sleeve 7 and adapter 9 in place, is inserted in the propellant perforation so that the flared end 8 of sleeve 7 is positioned at the opposite end of the propellant charge 5.

A closure member has cap 12, shown in Figure 5, is positioned over the end of the propellant charge 5 so as to leave a void space between the end of the propellant 5 and the inner surface of cap 12 which will be filled with bonding material so as to provide end restrictors for the propellent grain 5. Cap 12 also operates to maintain the support rods in position. A similar cap, having rod openings for removal of sleeve 7, is attached to the opposite end of the grain and these caps can be spaced apart by spacing rods which pass through the opening 13 of cap 12.

Side restrictors can be applied as strips and bonded to the propellent grain during the curing operation or the cap 12 can be constructed so that the bonding-restrictor agent can be applied similarly to the application of the end restrictors.

With the device assembled, the restrictor-bonding agent formulation is forced into the voids existing between the cap $12 and the propellent grain 5 and in the annular space 4 while the sleeve 7 is pulled up the support rod at a rate so that the flared end 8 provides resistance to flow through the radial drill holes covered by sleeve 7 thereby preventing the formation of voids in, the bonding agent. Thus, a combined operation permits the entire bonding agent and restrictor to be applied in a single step. After completion of the filling operation, the restrictor-bonding agent is cured before removal of the end caps 12 and thus the entire restrictor bonding agent is cured in a single step.

Various solid propellent compositions can be utilized as booster charges for rockets. Solid propellents contain a combination of oxidizer and fuel mixed together and formed into grains. The grains are usually formed by an extrusion process. Examples of solid propellent compositions include mixtures of nitrocellulose, nitroglycerinc and binder materials; mixtures of gun powder and carbon; mixtures of potassium perchlorate and asphalt; mixtures of ammonium picrate and sodium nitrate; and other combinations of oxidizers and fuels.

A preferred solid propellant for booster charges comprises grains of a mixture of ammonium nitrate, an organic, polymeric compound, such as a butadiene-methyl vinylpyridine copolymer, curing agents and a burning rate catalyst. The organic, polymeric compound acts as the fuel and as the binder for binding the ingredients into a solid grain. These propellants are produced by mixing the oxidant and copolymer together with suitable vulcanizing or quaternizing agents and a suitable burning rate catalyst. The resulting mixture is then molded or extruded and cured to form the propellent grain. These propellants and methods for their production are more fully described in copending application Serial No. 284,- 447, filed April 25, 1952, by W. B. Reynolds and J. E. Pritchard.

A particularly preferred propellent composition is prepared by combining from about 50 to 90 parts by weight of ammonium nitrate, as the solid oxidant, with to 50 parts by weight of binder and burning rate catalyst. The burning rate catalyst will usually comprise 0 to 10 parts by weight per 100 parts of the entire composition. i

The preferred binder will comprise about 25 to 98 parts by weight of 1,3-butadiene and about 2. to 75 parts by weight of Z-methyl-S-vinylpyridine. Burning rate catalysts include rouge, Prussian blue, Milori blue, and the like.

A wide choice of materials is possible for the restricting-bonding agents which can be utilized in the practice of this invention. Included are GRS-plasticizer-tackifier formulations, butadiene-methylvinylpyridine-plasticizertackifier formulations, natural rubber-solvent formulations, and various synthetic rubber-solvent formulations available as commercial rubber type bonding agents.

Suitable plasticizers include TP-90B, which is substantially composed of dibu-toxyethoxyethyl formal and is amples of the latter acids are phthalic acid and maleic acid. An especially good tricarboxylic acid for tackifier preparation is made by reacting rosin acid (abietic acid) with maleic anhydride. This acid is a polymerization product wherein the polymerization utilizes one double bond from each acid to form the polymer. In the class of resins which can be used as tackifier-hardeners are melamine-formaldehyde and urea formaldehyde resins modifiedby the addition of alcohols such as octyl alcohol. 1

These formulations are capable of being softened to the desired degree by the use of solvents such as benzene, toluene, xylene and other aromaticllydrocarbons and are made to cure under conditions compatible with the curing of rocket propellent grains by the addition of accelerators such as Bu-tyl-eight (a dithiocarbamate type rubber accelerator sold by R. T. Vanderbilt Company) and SA-113 (N,N-dimethyl-tertiary. butyl-thienyl dithiocarbamate) Dibutyl Phthalate- 1 LI 3 a liquid Thiokol rubber supplied by Thiokol Corporation. I ON-503 (now Araldite503) an epoxy type room temperature curing boding resin supplied by Oiba 00., Inc. 3 DMP-30 2,4,6 tri(Dimethylaminomethyl)phenol, Rohm & Haas. C Epon 828 an epoxy type resin manufactured by Shell Chemical ompany.

Ingredient GRS BdMYP GRSims Philblack A--- Pentaryl A Wood Rosin Stearic A Md 1.5 Flexamine 6 1.5 Zinc 0xide. 3. 0 3.0 Sulfur 1. 0.2 Butyl-eight 7 4. 0 Chloran i l 3. 0

1110-870 bonding agent by Minnesota Mining & Manufacturing. 5E8 iggi 5% Bonding agents by Products Research Corporation.

5 Pentaryl A=monoamyl biphenyl.

B Flexamine=N,N-diphenyl-p-phenylenediamine.

7 Buty1 eight=a dithioearbamate type rubber cure accelerator (R. T. Vanderbilt 00.).

The foregoing compositions are representative of the bonding materials which can be utilized in the practice of my invention. Those skilled in the art will be aware of other bonding compositions whichcan be utilized.

Reasonable variations and modifications are possible Within the scope of the disclosure of the present invention, the essence of which is the provision of a method and means for bonding a solid propellant grain to an internal support rod by providing peripheral openings in the support rod which are filled with bonding material supplied through a longitudinal opening extending through the support rod.

That which is claimed is:

I 1. A process for bonding a support rod to a perforated, external burning rocket grain so as to withstand the forces of acceleration which comprises forming said support rod with an axial opening therethrough and with a plurality of radial openings therethrough in cornmunication with said axial opening; positioning said rod in a rocket grain perforation so as to provide an annular space between said rod and said grain; positioning a close fitting impermeable sleeve around said rod so as to substantially occupy said annular space; filling said axial opening, and said radial openings of said rod with a rubbery'bonding agent with said sleeve in position; filling said annular space'with said bonding agent; and withdrawing said sleeve at substantially the same rate'that said annular space is filled whereby introduction of void space is substantially eliminated and curing said bonding agent. I 2. The method of claim lwherein said; bonding agent 75 is uncured natural rubber, curing agents and a plasticizer.

3. The method of claim 1 wherein said bonding agent is GRS rubber containing a plasticizer, a tackifier, a filler and an accelerator.

4. The method of claim 1 wherein said bonding agent is butadiene-methyl vinylpyridine rubber containing a plasticizer, a tackifier, a filler and an accelerator.

5. The method of claim 1 wherein said bonding agent is Thiokol rubber containing a filler and an accelerator.

6. A process for bonding a support rod to a perforated, external burning rocket grain so as to withstand the forces of acceleration which comprises forming said support rod with a longitudinal opening therethrough and with a plurality of peripheral openings in communication with said longitudinal opening and with the exterior of said rod; positioning said rod in a rocket grain perforation so as to provide an annular space between said rod and said grain; positioning a close fitting impermeable sleeve around said rod; filling all of said openings and said annular space with a resilient, uncured bonding agent while removing said sleeve from said rod and then ouring said bonding agent.

7. Means for bonding a perforated, external burning propellant grain to a support rod which comprises a support rod means adapted to be placed in the perforation in said grain and having an axial passageway therethrough and a plurality of radial passageways therethrough in communication with said axial passageway; means for positioning said rod in said perforation; means for introducing bonding material to said axial passageway; a removable sleeve means enclosing said rod and substantially filling the annulus between the rod and the grain for preventing flow of bonding material from said radial passageways until said sleeve is removed; and means for introducing bonding material into said axial passageway so as to substantially fill said annulus with bonding material as said sleeve is removed.

8. Means for simultaneously bonding a perforated, external burning propellant grain to a support rod and restricting the ends of said grain which comprises a support rod adapted to be placed in the perforation in said grain and having a longitudinal passageway therethrough and a plurality of radial passageways therethrough in communication with said longitudinal passageway; a pair of end closure means adapted to be placed over each end of said grain so as to position said support rod in place and to leave a void space between the end of said grain and the inner surface of said closure means; a sleeve means adapted for a close fit around said rod substantially filling the annulus between the rod and the grain for preventing flow of bonding material from said radial passageways until said sleeve is removed; and means for introducing bonding material into said longitudinal passageway so as to substantially till said annulus and said void space between the end of said grain and the inner surface of said closure means with bonding material as said sleeve is removed.

9. A process for bonding a support rod to a perforated, external burning rocket grain, so as to withstand the forces of acceleration, comprising a mixture of ammonium nitrate and a copolymer of butadiene and methyl vinylpyridine which has been formed into a grain and cured, which comprises forming said support rod with a longitudinal opening therethrough and a plurality of radial openings therethrough in communication with said longitudinal opening; positioning said rod in said rocket grain perforation so as to provide an annular space between said rod and said grain; positioning a close fitting impermeable sleeve around said rod so as to substantially occupy said annular space; filling said longitudinal opening and said radial openings of said rod with an uncured rubbery bonding agent with said sleeve in position; filling said annular space with said bonding agent and withdrawing said sleeve at substantially the same rate that said annular space is filled whereby introduction of voids is substantially eliminated, and curing said bonding agent.

10. A rocket propellant charge comprising a penforated, external burning propellant grain; a support rod having an axial passageway therethrough and a plurality of radial passageways therethrough in communication with the axial passageway, said support rod being positioned in the perforation of said grain so as to provide an annulus between said rod and said grain; and a resilient bonding material substantially occupying said passageways and said annulus and bonding said grain and said rod together into a unit.

References Cited in the file of this patent UNITED STATES PATENTS 2,548,926 Africano Apr. 17, 1951 2,724,864 Krotz Nov. 29, 1955 FOREIGN PATENTS 558,531 Great Britain Jan. 10, 1944 

